There's a diverse group of marine parasites, the myxozoans, that infect fish just by touching their gills or skin, so dietary preference alone wouldn't exclude

There's a diverse group of marine parasites, the myxozoans, that infect fish just by touching their gills or skin, so dietary preference alone wouldn't exclude them. Creepy little jellyfish cousins that evolved into parasites. And while the most well known ones are species specific (e.g., whirling disease in salmonids), they're more studied because they impacted aquaculture species, but we know of a fair few that can also infect multiple species. I'm wondering if there's been an environmental change in the water, like temperature or salinity or nutrient levels that has allowed parasites not previously found in the area to expand their range.